There has been significant commercial interest in multiple wavelength WDM systems in recent years, hereafter referred to as Dense Wavelength Division Multiplexing (DWDM) systems. These systems leverage the capability of a single optical fiber to carry multiple wavelengths of light, a technique that effectively multiplies the bandwidth capacity of a given fiber by the number of wavelengths that can be transmitted. The demand for higher bandwidth communication systems has been driven by large increases in information flow driven by the Internet and other data traffic. In addition, the invention of Erbium Doped Fiber Amplifiers (EDFA) to amplify the power levels of many wavelengths in a single amplifier has further increased the application space of this technology because the amplification cost per wavelength for an EDFA is very low compared to the Optical-to-Electrical-to-Optical (OEO) regeneration that would otherwise be needed, hence it is much cheaper to send large numbers of wavelengths over large distances in a single fiber than it would be to send individual wavelengths over separate fibers. This economic justification has led to the transmission of large amounts of bandwidth in multiple wavelengths over a single fiber, and in turn has driven the need to flexibly route those wavelengths at the wavelength layer to avoid bandwidth scaling issues with electrical based switching (cost/size/heat dissipation/upgradability) as well as the expense of OEO regeneration that electrical switching requires. The availability of cost-effective signal amplification from the EDFA has in part supported increased functionality at the wavelength layer by providing a reasonable insertion loss budget for new subsystems to obtain this functionality.
The earliest wavelength routing devices preceded the EDFA, and generally leveraged wavelength dependent evanescent coupling between multiple waveguides, or the wavelength-dependent transmission of a dielectric Thin Film Filter (TFF) to passively route light along multiple paths in a wavelength-dependent manner. In general these devices have low cost and low insertion loss, however the wavelength-dependent routing configuration is static and the intrinsic device provides for no control or feedback of the optical signals that flow through it. These limitations have been overcome in modern communication systems by designing subsystems from these basic wavelength-dependent routing building blocks that integrate detectors for monitoring, Variable Optical Attenuators (VOAs) for power level control, and optical switches for changing the routing configuration. This integration can be in the form of discrete devices (or arrays of devices), integration on a Planar Lightwave Circuit (PLC), or a hybrid free-space Wavelength Selective Switch (WSS).
The WSS is the newest and perhaps the most scalable wavelength monitoring and signal control device because it operates on all wavelengths within a single free-space region, thus has a lower per wavelength cost for many wavelength devices. The functionality of the device is shown in 100 of FIG. 1, where any wavelength on input 101 of the device can be routed via switches 104 to any output port 110-117 at a preset attenuation (or power level) determined by the settings of VOA 103, where that routing is independent of the routing of the other wavelengths because the input demultiplexer 102 and output multiplexer array (105) create independent routing paths for each wavelength. This is essentially the most flexible wavelength routing device possible, and it is achieved with better performance (insertion loss, filtering characteristics) than available from discrete components or integrated PLCs. Although this WSS technology is not yet mature, it seems quite likely that the premium performance and price of this technology will enable it to dominate signal control and monitoring applications in devices where flexible routing of many wavelengths is required.
At the present time, the segments of the communication network with economic justification for flexible routing of many wavelengths are the long haul, regional and/or metro-core networks. In the metro edge and or access networks there are currently not many wavelengths at a given node, and the traffic pattern of the wavelengths present is almost exclusively hubbed, that is information collected from all the edge nodes on a ring are backhauled to a single common hub node. The most efficient optical layer protection for this well-defined traffic pattern is a simple dedicated (1+1) protection scheme and essentially one end of every service is a-priori known to be the hub node. For these reasons, the per-wavelength routing flexibility of a WSS is not nearly as valuable as in other parts of the network where mesh traffic patterns prevail. In addition, this edge portion of the network is shared by the fewest customers, and hence requires the lowest cost structure. Therefore it is likely that even though the WSS is more cost-effective than the other technology alternatives with equivalent functionality, it is likely too expensive to use at the edge of the network where the flexibility it provides may not add sufficient value.
While network edge components may not require the extraordinary flexibility of a WSS, it still can benefit from added flexibility. The edge of the current network is dominated by the Multi-Service Provisioning Platform (MSPP), which aggregate multiple clients through an electrical switch fabric, and transmit a single wavelength line signal to the hub node (typically along two diverse paths). However as the bandwidth for a single customer approaches the capacity of a single wavelength, the services provided would transition to “wavelength services”, and numerous wavelength services might be required even within a single building that houses multiple businesses. Examples of these wavelength services are the full (non rate-limited) bandwidth services of Gigabit Ethernet (GbE) and 10 Gigabit Ethernet (10 GbE). This evolution of the network edge dilutes the value of MSPP electrical aggregation of many lower bandwidth signals into a single wavelength, while enhancing the value of DWDM networks that carry multiple wavelengths to a single access node. This type of network will typically be constructed from rings to provide signal protection through diverse path routing, and will have a hubbed traffic pattern such that traffic from all edge nodes is backhauled to the hub node. Today static TFF couplers largely service the DWDM Optical Add-Drop Multiplexing (OADM) at an edge node in this part of the network. These filters route (demultiplex) either a single wavelength or a band of wavelengths to a different output than the remaining DWDM wavelengths. Installation of new TFF filters breaks the optical path passing through a node, which is at the very least undesirable, and for many carriers unacceptable if signals from other nodes are interrupted. For this reason this part of the network frequently uses a TFF OADM that drop bands or groups of wavelengths from the express path. This approach has the benefit that only a single, relatively inexpensive and low loss TFF is needed at installation for the wavelengths passing through the node, thus adding additional drop wavelengths through that existing banded filter do not interrupt, or “hit” the existing services. Such a service addition does require installation of additional demultiplexing filters for each drop wavelength, however this installation is “hitless” because it does not impact the existing services.
There remains room for improvements of OADM flexibility in access applications even with the aforementioned benefits of TFF OADM. The need for improvement primarily stems from three limitations. The first is that for hitless upgrades of additional wavelengths the demands for each node need to be preplanned to correctly install the required TFF in the express path (through) at each node. This preplanning is not only time consuming, but also results in unused, stranded system capacity when capacity at that node does not materialize to meet the projected demand. The second limitation is the concatenation of multiple banded TFFs and individual demultiplexing drop filters is a time-consuming, manually intensive process that requires skilled craftsmen. The final limitation is this complex arrangement of filters can result in significant amounts of loss, including differential loss for different wavelengths. This loss variation results in a system reach that is wavelength-specific, implying wavelength-dependent engineering rules for the system that will be custom for every configuration. To mitigate this impact, systems typically install multiple individual attenuators on each transmitter to provide the required transmission performance for each channel. These latter two problems prevent the equivalent automation of signal routing in the optical layer as has always been provided in the electrical layer. This combination of non-automated, manual, and skill-intensive configuration procedures creates a barrier to fast deployment of DWDM at the edge of the network, and is in general an impediment to rapid deployment of cost-effective, high bandwidth services at the edge of the network.
Accordingly, it would be useful for a communication system to be cost optimized for the edge of the network while providing more automated line-side provisioning that does not require pre-planning while minimizing the probability that unused bandwidth will be stranded at the edge nodes. This application will describe such a system, including unique components and subsystems required for such a system.